1. Field of the Invention
The present invention relates to an ultrasonic flow meter for detecting the flow rate of a liquid based on a propagation velocity difference when acoustic waves are propagated through the liquid.
2. Description of the Related Art
Heretofore, an ultrasonic flow meter has been known in which, for example, paired transducers are arranged respectively on an upstream side and a downstream side of a conduit through which a liquid flows. Ultrasonic waves transmitted from one of the transducers are reflected by an inner wall surface of the conduit and are received by another of the transducers, and a flow velocity or a flow rate of the liquid is measured based on a difference in propagation velocities of the ultrasonic waves.
In such an ultrasonic flow meter, for example as disclosed in Japanese Patent No. 2793133 (Patent Document 1), a measurement conduit is provided having a supply pipe to which the liquid is supplied and a discharge pipe from which the liquid is discharged. Further, a first measurement head is disposed on one end of the measurement conduit, and a second measurement head is disposed on another end of the measurement conduit.
The first and second measurement heads are constituted to function as acoustic wave emitters or acoustic wave receivers. For example, a pulse shaped acoustic wave signal is transmitted from the first measurement head, and the second measurement head receives the acoustic wave signal as an acoustic wave receiver. Next, the first measurement head is switched to operate as a receiver, and by reception thereon of an acoustic wave signal transmitted from the second measurement head, the flow rate of the liquid is measured based on a difference in the propagation velocities of the acoustic waves.
However, with the ultrasonic flow meter according to the above-described conventional technique, a structure is provided in which the liquid supplied to the supply pipe is changed in direction substantially perpendicularly, whereupon the liquid then flows to the measurement conduit. Therefore, by the sudden change in the flow direction of the liquid, a pressure change occurs resulting in turbulence, so that air entrained in the liquid forms gas bubbles in the liquid, whereby such gas bubbles adhere to the inner wall surface of the measurement conduit, which is disposed in confronting relation to the first and second measurement heads. Due to adhesion of such gas bubbles, the gas bubbles interfere with propagation of the acoustic wave signals, leading to deterioration in the accuracy with which the flow rate of the liquid is measured.
Thus, with the aim of preventing adhesion of gas bubbles as described above, in the ultrasonic flow meter disposed in Japanese Laid-Open Patent Publication No. 2010-243245 (Patent Document 2), surface processing is carried out on an inner circumferential surface of a measurement tube through which a liquid flows, such that wettability is increased upon flowing of the liquid, and together therewith, adherence of gas bubbles with respect to the inner circumferential surface is prevented.